Astronomical and Nautical Collections, 203 



gatlon, or to the lengths of undulation of the light in the 

 two mediums, as will shortly be seen in the demonstration of 

 the laws of refraction. 



Having explained the formation of the reflected rings by 

 the interference of the reflections from the first and second 

 surface of the plate of air, Dr. Young has shown, that the 

 much weaker rings, which are seen by transmission, result 

 from the interference of the rays directly transmitted with 

 those which have first been twice reflected within the thin 

 plate ; and that they must naturally be complementary to the 

 reflected rings, as experiment proves that they are. It is un- 

 necessary to enter into the detail of their explanation at pre- 

 sent ; it must only be remembered, that the great faintness 

 of these rings, when the incidence is perpendicular, depends 

 on the great dift'erence of intensity of the two portions of 

 light : [and more especially on the effect of the unaltered 

 light, which is inseparable from them, in effacing their im- 

 pression on the eye ; for there is reason to think their inten- 

 sity more considerable than Dr. Young at first supposed. 



Tr.] 



We may also pass over the phenomena of rings exhibited 

 in light falling obliquely on the plates : observing only that 

 the theory explains the circumstance of the augmentation of 

 their diameters as the obliquity increases, and that the very 

 simple formula, which is derived from it, represents the phe- 

 nomena with perfect accuracy, at least until the obliquity 

 becomes extremely great : for when the light is very much 

 inclined to the surfaces, the results of the calculation are not 

 so perfectly conformable to the measurements of Newton. 

 But it is probable, that this irregularity depends upon some 

 deviation from the ordinary laws of refraction, which are 

 supposed in the calculation, in the case of light passing so 

 obliquely between surfaces so near to each other. 



The rings, v/hich we have hitherto considei'ed, are such as 

 are produced by homogeneous light : but it is easy to infer 

 from them what must happen in white light, by considera- 

 tions similar to those which have been employed in the case 

 of fringes, exhibited by two mirrors. This analysis of the 

 phenomena may be found very mu^h at length in the Optics 



